Lithium ion secondary batteries, which are now widely used as power supplies for mobile electronic equipment, have achieved high potential and high capacity by employing, as active materials for negative electrodes, carbon materials capable of inserting and extracting lithium and by employing, as active materials for positive electrodes, composite oxides of transition metal and lithium, e.g. LiCoO2. However, further increase in capacity is required with an increased range of functions of recent electronic equipment and communication equipment.
For example, to increase the capacity of a secondary battery, an electrode group in which a positive electrode plate and a negative electrode plate are wound with a separator interposed therebetween is pressed so that the space occupied by the positive and negative electrode plates in a battery case increases.
However, when the electrode group is pressed to hardly leave a gap between the electrode plates and the separator, it disadvantageously takes a long time to impregnate the entire electrode group with an electrolyte after pouring the electrolyte into the battery case. This problem is conspicuous especially for nonaqueous electrolytes used for lithium ion secondary batteries because of high viscosity of the nonaqueous electrolytes.
In view of this, Patent Documents 1 and 2 disclose methods for enhancing impregnating ability for electrolytes by forming trenches in the surfaces of mixture layers of electrode plates.
FIG. 7 is a view illustrating a structure of an electrode plate disclosed in Patent Document 1. In FIG. 7, a plurality of trenches 102 are formed in the surface of a mixture layer 103 of an electrode plate 101. The electrode plate 101 is sandwiched between separators 104, thereby forming an electrode group. An electrolyte poured into a battery case permeates the entire electrode group by way of these trenches 102, thus reducing the time for impregnation. When the width and depth of the trenches 102 are increased, the impregnation time is reduced. However, the amount of the mixture layer decreases in this case, so that charge/discharge capacity decreases. In view of this, the width and depth of the trenches 102 are set at given values.
The trenches formed in the surface of the electrode plate can cause breakage of the electrode plate during formation of an electrode group by winding the electrode plate. To prevent this, a method for preventing breakage of an electrode plate while enhancing impregnating ability is disclosed in Patent Document 2.
FIG. 8 is a view illustrating a structure of an electrode plate disclosed in Patent Document 2. In FIG. 8, a plurality of trenches 112 are formed in the surface of an electrode plate 111 to be inclined from the longitudinal direction of the electrode plate 111. In the formation of an electrode group by winding the electrode plate 111, tension is applied in the longitudinal direction of the electrode plate, which is different from the direction along which the trenches 112 are formed. Accordingly, tension on the trenches 112 is dispersed, thus preventing breakage of the electrode plate 111.
Patent Document 3 discloses a method for forming a plurality of trenches in the inner surface of a wound electrode plate for the purpose of not enhancing impregnating ability for an electrolyte but preventing deformation of the electrode plate during formation of an electrode group by winding the electrode plate.
FIG. 9 is a view illustrating a structure of an electrode group disclosed in Patent Document 3. In FIG. 9, a plurality of trenches 122 are formed in the inner surface of a wound electrode plate 121. This structure reduces pressure on the inner surface of the electrode plate 121 during winding of the electrode plate 121, thus preventing deformation of the electrode plate 121.    Patent Document 1: Japanese Domestic re-publication of PCT international application No. 98/048466.    Patent Document 2: Japanese Laid-Open Patent Publication No. 11-154508    Patent Document 3: Japanese Laid-Open Patent Publication No. 8-153515